Processing of thermosetting materials by methods of injection-moulding or compression-moulding with a pre-plastication with a screw imposes tightened requirements on the invariability of their granulometric composition.
Manufacture of granulated composite thermosetting plastics is performed in two ways, namely: either during production of composite materials or thereafter, i.e. before processing to articles.
Known in the art is a method of granulation of thermosetting plastics by extrusion which comprises melting melamine or phenolic resins in an extruder and extrusion of the resulting melt through a grate at a temperature within the range of from 80.degree. to 100.degree. C.; the thus-produced strands are cut to granules of a predetermined size.
However, the difficulties associated therewith due to a high accuracy of maintaining temperature conditions substantially hinder the extrusion process. Furthermore, the resulting granules do not possess sufficiently good injection-moulding properties.
"Werner Pfleiderer", a West German company, has attempted to overcome this disadvantage by developing a continuous process for the manufacture of a thermosetting composite material and granulation thereof by extrusion. In this process the starting mass is melted, homogenized and partly condensed, whereafter it is added with an "agent" stopping the condensation reaction and lowering the product viscosity. As such "agent" water is used in an amount of from 0.5 to 8.0% by weight.
It is also known, however, that the addition of water into a composite thermosetting material impairs dielectric and physico-mechanical properties of articles further manufactured from this material.
In the extrusion of a thermoplastic material the rate of outflow of the extrudate is limited by the formation of various defects on the surface thereof which result in an impaired appearance of the final articles. Furthermore, great difficulties are encountered in extrusion processing of high-viscosity thermoplastic materials.
Known in the art is a process for the manufacture of articles from thermoplastics such as polyolefins with the melt index of 0.3 g/10 minutes which involves intermixing of the polymer with a lubricating agent and subsequent extrusion in a piston extruder. The process of the manufacture of final moulded articles is performed in the following manner. Granules of a polymer material from a charging bin are fed into an extrusion channel, wherein said granules are softened under the effect of temperature and become molten and then, while passing via cooling zone, are moulded into articles. Mouldability of the thermoplastic material in this case is ensured by the addition of lubricating agents thereinto such as glycerol with amides of higher aliphatic acids. This prior art process however, has certain disadvantages residing in a low moulding productivity (the outflow rate of a final article ranges only from 0.15 to 0.64 m/min) and a low degree of densification of the material in the forming die which results in an impaired quality of the final article. Moulding of highly extended thermoplastics or thermoplastics containing a large amount of a gel-like fraction is totally impossible to perform using this method, i.e. extrusion, even with the addition of lubricating agents.